In an IP-based real-time communication system, such as a TV conference system, full high-definition (Full HD) video transmission is already possible. In an IP network environment, a packet loss of about several percent occurs. Accordingly, video or audio quality is ensured using a retransmission (Automatic Repeat reQuest: ARQ) or packet loss correction (Forward Error Correction (FEC)) function. In the case of a real-time video transmission system, the ARQ is effective in the environment where the Round Trip Time (RTT) is short, and the FEC is effective in the environment where the RTT is long or the environment where the packet loss rate is high.
In recent years, a video transmission system in a mobile environment has appeared with an increase in the speed of wireless channels, such as WiMAX or Long Term Evolution (LTE). Since a phenomenon such as handover or changes in the wireless environment due to the movement of a terminal occurs frequently in a wireless environment, the packet loss rate or the RTT tends to be worse than that in a wired environment. Accordingly, the FEC is suitable as a method of ensuring the video or audio quality in the mobile environment. In the FEC, redundant packets (hereinafter, also referred to as “parity”) are generated from data packets of a video or audio, and are transmitted on a network together with the data packets. In a wireless environment where the transmission bandwidth is limited, however, the transmission bandwidth of data packets is compressed due to the overhead of redundant packets, and this may have an adverse effect on the video or audio quality. Therefore, system design to ensure a certain degree of quality while reducing the amount of redundancy is required.
Patent Literature 1 discloses a transmission-side configuration in a video transmission system. FIG. 17 is a block diagram showing configuration examples of a transmission apparatus and a reception apparatus that configure a communication system disclosed in Patent Literature 1. A rate control unit 501 provided in a transmission apparatus 500 determines the total transmission rate from network information (packet loss rate, RTT, and the like) notified from an RTCP unit 211, and notifies a redundancy determination unit 503 of the result. In addition, the RTCP unit 211 notifies the ARQ unit 210 of NACK information from a reception apparatus 300. The redundancy determination unit 503 calculates an adjustment rate, which can be used in the actual data transmission, from the total transmission rate notified from the rate control unit 501 and the retransmission data size notified from the ARQ unit 210, and notifies an encoder 202 of the result. In addition, the redundancy determination unit 503 determines the number of redundant packets using a redundancy table shown in FIG. 18 that is configured to contain the RTT and an adjustment rate obtained by subtracting the retransmission data rate from the total transmission rate, and notifies an FEC encoder 205 of the result.